This invention relates to the recovery of high boiling gaseous compounds such as vaporized, normally liquid compounds, from gaseous mixtures, and more particularly to the recovery of vaporized solvents from solvent-inert carrier gas mixtures by condensing the solvent from the gas mixture.
Many technical and industrial processes result in the production of a product or by-product gas stream that contains a vaporized, normally liquid compound, i.e. a compound that is liquid at normal temperatures and pressures. For example, in the curing of organic solvent-borne resin coatings that have been deposited onto surfaces or articles, e.g. painted or lacquered metal parts for automobiles, home appliances and the like, it is common to bake the resin coating in an oven, thereby evaporating the solvent contained in the coating mixture and curing the solvent-free resin coating material. During the drying process the vaporized solvent is generally removed from the drying oven by a carrier gas that is inert with respect to the vaporized solvent under conditions that prevent or minimize the possibility of formation of an explosive or flammable gas mixture.
There is often a need to recover the high boiling gas component from the above-described gas mixtures. For instance, in some cases, the high boiling gas may have economic value that makes its recovery very desirable. In other cases, the high boiling gas may be hazardous or harmful to the environment and thus cannot be released to the atmosphere. Various methods have been employed for effecting the recovery of high boiling gas components from gas mixtures. Typical methods include condensation, adsorption and membrane separation. Condensation is often preferred because of its simplicity and efficiency.
Separation by condensation involves cooling the high boiling gas-laden gaseous stream to a temperature well below its dew point, thereby causing the high boiling gas to condense to the liquid state. The liquefied gas is then removed from the system. Processes for condensing high boiling gases from gas streams are disclosed in U.S. Pat. Nos. 4,122,684, 4,188,793, 4,237,700, 4,444,016, and 4,545,134.
Condensation works well for the recovery of high boiling gaseous compounds. Gas mixtures containing these compounds can be cooled to a temperature low enough to remove sufficient high boiling gas from the stream to enable the lean gas stream to meet the strict environmental standards set for gases discharged to the atmosphere without freezing the high boiling gas component on the surfaces of the condenser in which the gas mixture is cooled. However, when the gas stream contains contaminants which freeze at or above the temperature at which the heat exchange equipment must be operated, such as water vapor and/or carbon dioxide, it is difficult to successfully use a single condenser to condense the high boiling gas, since the condenser will eventually clog up with the frozen contaminant and have to be taken out of service until the frozen contaminant is removed from the condenser.
U.S. Pat. No. 4,188,793, issued to Watson et al. discloses the removal of impurities such as water and carbon dioxide from a gas mixture from which vinyl chloride is also to be separated by passing the incoming gas mixture through heat exchangers in which the impurities are frozen. Reversing heat exchangers can be employed in alternate service, so that while the contaminant is being frozen in one heat exchanger, the frozen contaminant is being thawed in the other heat exchanger. This patent also discloses removing vinyl chloride from the dried gas stream by condensation using liquefied nitrogen as the refrigerant. The nitrogen, which is vaporized and warmed during the condensation step is subsequently used to thaw the frozen impurities.
It is also known to pass the gas stream through adsorbers to remove moisture and other condensable contaminants from the gas stream prior to introducing the gas stream into a solvent condenser. The adsorbers are conventionally regenerated by passing a heated gas obtained from an external source, such as dry nitrogen or air, through the adsorbers. The solvent-depleted gaseous effluent from the condenser is usually recycled to the source of the gas stream for reuse as an inerting and carrier gas. When the gas stream contains a noncondensable impurity, such as oxygen or carbon monoxide, which enters the system as a result of leakage or chemical reaction, there is a likelihood of undesirable build-up of the impurity in the gas stream. Accordingly, it is not always practical to reuse the condenser effluent as a carrier gas, and fresh inert carrier gas must be used. This reduces the operating efficeincy of the recovery system.
There is a continuing search for improved systems for the recovery of high boiling gases from gas streams that additionally contain non-condensable gaseous impurities, the build-up of which cannot be tolerated in the system being treated. In particular, it is desirable to develop improvements to such systems that will enable them to operate more efficiently and effectively and at lower costs. The present invention provides improvements which produce these benefits.